The present invention is directed to a thermoplastic molding composition and more particularly to a transparent blend that contains polycarbonate resin and atactic copolymer of PMMA.
A thermoplastic molding composition is disclosed containing polycarbonate resin and a copolymer of methyl methacrylate conforming structurally to: 
wherein x and y are integers calculated to result in a content of PMMA in the copolymer in the range of 80 to 90 mole %, and where R1 denotes xe2x80x94CH3, R2 denotes xe2x80x94C6H5 and R3 is a C1-C2-alkyl group.
The composition is useful in preparing transparent articles, including films.
Polycarbonate resins are characterized by dimensional stability at relatively high temperatures, excellent resistance to impact, stiffness and transparency. These properties make polycarbonate the material of choice for a variety of applications including glazing containers and medical devices. A notable drawback characterizing this resin has been its susceptibility to scratching. Polymethylmethacrylate (xe2x80x9cPMMAxe2x80x9d), known for its clarity and scratch resistance is noted for its shortcomings in terms of dimensional stability, low impact strength and relatively poor thermal stability. While some blends of polycarbonates and PMMA reflecting a more attractive profile of properties are known, it is also known that a wide range of blends with typical PMMA are immiscible and opaque. It has been suggested that free radical polymerized PMMA does not form a single thermo-dynamically miscible transparent blend but does demonstrate mechanical compatibility with polycarbonate. While this compatibility is taken to mean that the phases exhibit good adhesion one to the other, the resulting blend is not transparent.
Literature relative to mixtures of polycarbonate and PMMA include U.S. Pat. No. 4,319,003 that disclosed opaque blends. Also noted in this connection are JP 7216063 and EP 297285. Ways of overcoming the drawbacks associated with the immiscibility of PMMA and polycarbonate have been proposed. These include the addition of copolymer additives (DE 2264268); PMMA/acrylamide copolymers (DE 3632946 and PMMA-ester copolymers containing carboxylic groups (U.S. Pat. No. 4,906,696). Further, transparent blends of PMMA and polycarbonate are said to have been prepared, in accordance with DE 3,833,218, by melting the two components in the presence of supercritical gas. U.S. Pat. Nos. 4,743,654 and 4,745,029 disclosed producing solutions of the two polymers in organic solvents and evaporating the solvents as means to prepare transparent material. Transparent films of polycarbonate and polymethylmethacrylate produced by special solvent-removal methods, however, tend to become opaque at temperatures above 145xc2x0 C. due to the immiscibility of these polymers. Mixtures of polycarbonate and stereoregular polymethyl methacrylate in which at least 60% of the monomer units are in the syndiotactic configuration have been disclosed in EP 573,109. U.S. Pat. No. 5,284,916 is noted for disclosing a blend of polycarbonate and a block copolymer containing a polyaromatic(alkyl)-methacrylate block.
The present invention concerns a thermoplastic molding composition containing polycarbonate and a specifically structured copolymer of methyl methacrylate (herein xe2x80x9cCOPMAxe2x80x9d). The composition, containing about 5 to 20, preferably 7 to 15 percent of a suitable COPMA, the percent being relative to the total weight of polycarbonate and COPMA is characterized by its transparency. The inventive transparent blend may be processed thermoplastically in accordance with conventional procedures and using conventional means.
Aromatic polycarbonates within the scope of the present invention are homopolycarbonates, copolycarbonates, branched polycarbonates and mixtures thereof. The polycarbonates generally have a weight average molecular weight of 10,000 to 200,000, preferably 20,000 to 80,000 and their melt flow rate, per ASTM D-1238 at 300xc2x0 C., is about 1 to about 65 g/10 min., preferably about 2 to 15 g/10 min. They may be prepared, for example, by the known diphasic interface process from a carbonic acid derivative such as phosgene and dihydroxy compounds by polycondensation (see German Offenlegungsschriften 2,063,050; 2,063,052; 1,570,703; 2,211,956; 2,211,957 and 2,248,817; French Patent 1,561,518; and the monograph H. Schnell, xe2x80x9cChemistry and Physics of Polycarbonatesxe2x80x9d, Interscience Publishers, New York, N.Y., 1964 (all incorporated herein by reference).
In the present context, dihydroxy compounds suitable for the preparation of the polycarbonates of the invention conform to the structural formulae (1) or (2): 
wherein
A denotes an alkylene group with 1 to 8 carbon atoms, an alkylidene group with 2 to 8 carbon atoms, a cycloalkylene group with 5 to 15 carbon atoms, a cycloalkylidene group with 5 to 15 carbon atoms, a carbonyl group, an oxygen atom, a sulfur atom, xe2x80x94SOxe2x80x94 or xe2x80x94SO2xe2x80x94 or a radical conforming to: 
e and g both denote the number 0 to 1; Z denotes F, Cl, Br or C1-C4-alkyl and if several Z radicals are substituents in one aryl radical, they may be identical or different from one another; d denotes an integer of from 0 to 4; and f denotes an integer of from 0 to 3.
Among the dihydroxy compounds useful in the practice of the invention are hydroquinone, resorcinol, bis-(hydroxyphenyl)-alkanes, bis-(hydroxyphenyl)-ethers, bis-(hydroxyphenyl)-ketones, bis-(hydroxyphenyl)-sulfoxides, bis-(hydroxyphenyl)-sulfides, bis-(hydroxyphenyl)-sulfones, and xcex1,xcex1-bis-(hydroxyphenyl)-diisopropylbenzenes, as well as their nuclearalkylated compounds. These and further suitable aromatic dihydroxy compounds are described, for example, in U.S. Pat. Nos. 3,028,356; 2,999,835; 3,148,172; 2,991,273; 3,271,367; and 2,999,846, all incorporated herein by reference.
Further examples of suitable bisphenols are 2,2-bis-(4-hydroxyphenyl)-propane (bisphenol A), 2,4-bis-(4-hydroxyphenyl)-2-methylbutane, 1,1-bis-(4-hydroxyphenyl)-cyclohexane, xcex1,xcex1xe2x80x2-bis-(4-hydroxyphenyl)-p-diisopropylbenzene, 2,2-bis-(3-methyl-4-hydroxyphenyl)-propane, 2,2-bis-(3-chloro4-hydroxyphenyl)-propane, bis-(3,5-dimethyl4-hydroxyphenyl)-methane, 2,2-bis-(3,5-dimethyl-4-hydroxyphenyl)-propane, bis-(3,5-dimethyl-4-hydroxyphenyl)-sulfide, bis-(3,5-dimethyl-4-hydroxyphenyl)-sulfoxide, bis-(3,5-dimethyl-4-hydroxyphenyl)-sulfone, dihydroxybenzophenone, 2,4-bis-(3,5-dimethyl-4-hydroxyphenyl)-cyclohexane, xcex1,xcex1xe2x80x2-bis-(3,5-dimethyl4-hydroxyphenyl)-p-diisopropylbenzene, 2,2,4-trimethyl cyclohexyl 1,1-diphenol and 4,4xe2x80x2-sulfonyl diphenol.
Examples of particularly preferred aromatic bisphenols are 2,2,-bis-(4-hydroxyphenyl)-propane, 2,2-bis-(3,5-dimethyl-4-hydroxyphenyl)-propane, 2,2,4-trimethyl cyclohexyl 1,1-diphenol and 1,1-bis-(4-hydroxyphenyl)-cyclohexane.
The most preferred bisphenol is 2,2-bis-(4-hydroxyphenyl)-propane (bisphenol A).
The polycarbonates of the invention may entail in their structure units derived from one or more of the suitable bisphenols.
The copolymer of methyl methacrylate of the present invention (herein COPMA), conforms structurally to: 
wherein x and y are integers calculated to result in a content of PMMA in the copolymer in the range of about 80 to 90 mole %, R1 denotes xe2x80x94CH3, R2 denotes xe2x80x94C6H5 and R3 is hydrogen or a C1-C2-alkyl group.
The preparation of the inventive COPMA may be carried out by free radical polymerization of methyl methacrylate in the presence of an appropriate vinyl-benzoate comonomer.
Suitable benzoate monomer may be synthesized from xcex1-methyl-para-hydroxystyrene with benzoyl chloride in the presence of an amine base (such as triethylamine). In the course of the work leading to the present invention, 4-phenol ispropenylbenzoate was thus prepared. 